Splicing optical fibres is often made more difficult by the fibre core not lying centrically in the fibre cladding. In such a case, when two fibre ends which are centered starting from the outer surface of the cladding are welded together, the cores will be displaced in relation to each other, resulting in that the splice heavily attenuates a transmitted light signal. This problem is particularly evident in so-called single mode fibres which have a very narrow core and a comparatively very heavy cladding. To enable good centering, a light signal may be sent from one fibre through the contact location of the fibres to the other fibre where the light signal is detected as described, for example, in the British patent application No. 2115948. This publication describes an apparatus where the fibre is bent with a small radius of curvature in the vicinity of the contact location, so that light may be coupled into and out from the fibre at the bent places. The apparatus has the disadvantage that the basic modes of the light as well as higher order modes are detected during the centering process. This can result in that the fibres are kept mutually laterally displaced in spite of the detection, so that an incorrect splice is obtained.